This invention relates to apparatus for receiving and executing commands transmitted by means of a modulated carrier signal, and more particularly to apparatus of this type, such as load control receivers, for performing load management functions at a user's location.
A load control receiver is a device used at a customer site to perform load management functions for example to interrupt and reconnect power to customer loads upon command. Such loads are, for example, water heaters, air conditioners, heat pumps, and the like. Typically, commands to interrupt and reconnect such loads are generated at a substation by a substation control unit. The commands are preferably sent utilizing a carrier signal which is transmitted over the power distribution lines which extend from the substation to the user's facility.
Information is impressed on the carrier preferably using a differential phase shift keying (DPSK) modulation technique which, in a high noise environment such as that existing on the power distribution lines, places the interfering signals (noise) in the nulls or orthogonalities of the detector. DPSK detection utilizes the phase of the signal as detected in each baud interval for the phase reference of the signal to be detected in the following interval. This establishes a new phase reference during each baud period. Carrier phase shifts can be many times faster than the message rate and still be properly received by a DPSK receiver. Since high noise periods nearly always coincide with high load periods, it is desirable that such a reliable transmission technique be employed to insure that load control functions and time of day rate changes are accurately carried out at these times.
Typical prior art DSPK demodulators are shown in U.S. Pat. No. 4,225,964 issued to Cagle and Weeren. Such DPSK demodulators utilized four sets of analog integrators and accompanying digital to analog circuitry to perform the required integrations. Such prior art systems typically utilized operational amplifier integrators to integrate the inphase and quadrature reference signals at the carrier frequency. The section of the received signal waveform being integrated varies according to the phase relationship between the received signal and the inphase or quadrature reference signal driving the integrator. The outputs of the four analog integrators are multiplexed and supplied to one input of a voltage comparator. The other input of the voltage comparator is supplied by the analog output of a digital to analog converter.
The output of the digital to analog converter is compared to the output of the addressed integrator circuit. The output of the voltage comparator is a logic one if the integrator output is less positive than the digital to analog converter output. The comparator output is a logic zero if the integrator output is more positive than the converter output. The "one-zero" output of the voltage comparator is applied to an input of a modem microprocessor which, in turn, supplies an eight bit address that controls the analog output of the digital to analog converter. In brief, the system uses a digital to analog converter and a voltage comparator to perform an analog to digital conversion on the integrator outputs. The result is a digital word representing the inphase or quadrature component of the message bit.
Although such analog systems perform satisfactorily, they require a relatively large amount of hardware and are correspondingly relatively expensive to manufacture.
It is therefore an object of the present invention to provide an apparatus for demodulating commands transmitted by means of a differential phase shift keyed signal, which apparatus comprises relatively fewer components than the above described prior art apparatus.
It is another object of the present invention to provide an apparatus for reliably receiving commands over a noisy transmission system, which apparatus is relatively inexpensive to manufacture.
It is yet another object of the present invention to provide an apparatus for demodulating commands which have been modulated onto a carrier using differential phase shift keying techniques and transmitted over electrical power distribution lines, which apparatus comprises fewer components than previously utilized in the comparable apparatus described hereinabove.
It is still another object of the present invention to provide an apparatus for demodulating commands sent over a power line distribution system utilizing a differential phase shift keying modulation technique, which apparatus utilizes digital demodulation techniques.